1. Field of the Invention
This invention relates to a pulping process and more particularly to utilization of the combination of ammonia and various ketones in the cooking liquor of a pulping process.
2. Prior Art
The problem of increasing yield without appreciably decreasing quality of the pulp is a major problem facing the pulp industry today. Yield is the weight percentage of oven-dry cellulosic raw material recovered as pulp at the end of the pulping process.
The cellulosic raw material in wood has three principal constituents: celluloses, hemi-celluloses and lignins. Pulp that is high in quality, having good strength and bleachability, has two of these constituents -- cellulose and hemi-cellulose -- and little lignins. Therefore, for many products, it is desirable to have a pulp process that will dissolve only lignins from the cellulosic raw material and leave the cellulose and hemi-cellulose unharmed. This process would produce a high yield-high quality pulp.
The most widely used chemical pulping process is the kraft process. Basically, the kraft process is an alkaline cooking process in which sodium hydroxide and sodium sulphide are used to digest cellulosic raw materials. The kraft process, being highly alkaline, dissolves, in addition to the lignin, a significant part of the hemi-cellulose and some of the cellulose by decomposition reactions. This produces a low-yield pulp which is characterized by fibers having good strength and fair bleachability.
This low yield from the kraft process affects the economy of the pulping process in many ways. First, a larger quantity of wood is required to produce a given quantity of pulp. Second, a larger quantity of spent liquor solids have to be processed. Further, the kraft process produces pollutants in the form of odorous sulphur compounds and particulates in the micron and submicron range. The complete elimination of these pollutants is costly and requires the utilization of sophisticated chemical recovery systems. Further, the presence of these sulphur compounds causes increased corrosion of the pulping equipment. Still today, the kraft process produces a pulp which can be used in a wider variety of products than any other.
Other processes have been developed to give products having different characteristics, notably the acid sulphite and neutral sulphite processes. Although the pulp obtained by the acid sulphite process has good bleachability, it does not have the strength of kraft and is also obtained at a relatively low yield.
Neutral sulphite pulp is produced at higher yields. However, the residual lignin content of this pulp is too high for practically all applications except corrugating medium. Neither acid nor neutral sulphite processes pulp resinous woods such as Douglas fir and pines well.
Ammonia base sulphite and bisulphite processes are also used for pulping. U.S. Pat. No. 2,032,437 describes on page 2, lines 1-10, that ammonia gas may be added to the cooking liquor.
U.S. Pat. No. 1,880,047 describes the use of ammoniated water to prepare refined cellulose pulps from crude pulp. U.S. Pat. No. 2,070,585 describes the use of organic solvents, including ketones or mixtures of organic liquids of different polarities, to extract ligno-cellulosic materials to produce cellulose. U.S. Pat. No. 2,037,001 describes the use for pulping of alcohols which form with water homogeneous mixtures under the hotter conditions for digesting, and which separate into immiscible layers upon cooling after the cooking process is complete. It also describes, on page 2, the use of small quantities of urea to neutralize the acids formed in the reaction. It notes that an ammonia odor is distinctly detectable. A table on page 4 compares the digestion with and without urea. Urea has also been used with sulphite cooks. This is disclosed in U.S. Pat. No. 3,368,935.